The present invention relates to a shut-off device with progressive opening that can be used in particular in a throttle valve to regulate the rate of flow of air to an internal-combustion engine and thus to determine the power it develops, by the movement of a part--typically in the form of a disc--that can pivot about one of its diameters perpendicular to the axis of the pipe in which it is fitted.
In many cases, and in particular in the throttle valve mentioned above, it is important to be able to regulate the rate of flow of a fluid in as progressive a manner as possible, especially at low flow rates. However, in the case of low-viscosity fluids such as air, most shut-off devices have a virtually "all-or-nothing" action, or at any rate are incapable of precise and progressive regulation of low flow rates.
In addition to a satisfactorily progressive control of flow rate, it is beneficial for such shut-off devices to have at least some measure of adaptability, in order that one device can display different flow characteristics and can thus be used in different contexts (to allow it to be fitted, say, to engines of different horsepower), with the least possible adaptation work. In other words it is desirable that the majority of the components of the device be common to its different applications, and that any parts specific to each of these applications be as few and as inexpensive as possible.
Most known throttle valves are made of metal, for example aluminium, so that the tolerances--and consequently the flow characteristic--are not excessively affected by the large temperature variations to which such a device can be subject, especially in a vehicle, and are also reasonably identical from one specimen to the next. The use of metal is an expensive solution; it would be preferable to find a solution permitting the use of plastic materials, which would also offer a reduction in the weight of the device.
Proposed solutions have already been put forward to solve these problems. For example, document GB 2,245,931 proposes the use of a plastic shut-off member of complicated shape (FIG. 8) comprising in particular a profiled lip (145) to reduce the flow rate when the angle of opening is small. It goes without saying that the production of such a device is complex and demands great precision. This usually means using high-performance--and therefore high-cost--plastic materials, or the use of machining. In addition, the device cannot be adapted easily to different types of engines without machining or replacing the shut-off member. Another problem with the device disclosed in this document is that the profiled lip running all the way around the periphery of the shut-off member, and therefore across the full width of the pipe, creates a significant loss of pressure when the shut-off member is fully open. The device is also characterized by an extremely complicated and bulky bypass idle system.
Another proposal for achieving progressive control of low flow rates involves giving the inside surface of the section of pipe in which the shut-off member is installed an approximately spherical shape so that when the shut-off member pivots open, the distance separating its edge from the wall (and therefore the flow rate of the fluid) initially rises very progressively in a predetermined manner. This proposal, however, makes the pipe section difficult to fabricate, owing among other things to its undercuts. Moreover the precision with which the inside surface of the pipe must be profiled makes the use of plastic materials inappropriate.